Telescopic hoists consist of a series of telescopically arranged tubular sections with a cap closing a first end of each section. A second end of each section is mounted with a two-piece cylinder head while an innermost tubular section has a plunger pin eye which threads into the tube section. Such hoists are hydraulically operated to move the tube sections telescopically. The cylinder heads are threadedly mounted to an outer wall at the second end of each section; they are provided with dynamic and static seal means for sealing and with wiper means for removing debris from a surface along which the dynamic seal means slidably contacts.
On the one hand, rod seal type cylinders are known in the art, such as the one described by Mott in U.S. Pat. No. 4,003,297 issued on Jan. 19, 1977, wherein a sealing ring slides on the exterior diameter of each tubular section. This sealing ring as well as the wiper are located in the piston head. In the cases of double action hoists, sealing rings are located on the piston and these sealing rings are not wipers and are subject to contamination. Such a cylinder is a system closed to the atmosphere, so that air cannot enter into the cylinder at each run. The use of a wear ring on each side of the piston is to avoid wear between the piston and the interior wall of the tube. Certain types of wear rings are made of plastic material, which results in some particles to be embedded in the plastic material. The wear ring is mainly an antifriction component, which offers very limited protection against contamination of sealing rings located on the piston.
On the other hand, bore seal type cylinders are known in the art, wherein where the sealing ring slides on the internal diameter of each tubular section. For example, Dawson, in U.S. Pat. No. 5,983,778 issued on Nov. 16, 1999 discloses a single action cylinder that opens when it is fed with hydraulic oil through an inlet 28 located at its base and closes under the load effect applied on its last section. It is an open system, which means that the ambient air penetrates inside the cylinder, by the provision of a breather on each hoist in order to enable ambient air to be introduced into the cylinder. In practice, this breather is very quickly filled which results in causing air to be pushed and aspired through the wipers installed on these piston heads. These wipers therefore are very rapidly damaged thereby leaving debris to contaminate the hoist. Furthermore, the Dawson patent is concerned with providing a telescopic hydraulic hoist made from rolled aluminium stock and it is submitted that aluminum does not have adequate mechanical features to guarantee the structural integrity of a hoist when submitted to high operating pressures (2200 p.s.i.).